QLCS tornadoes: the ones a squall line spins up

A QLCS tornado is a tornado produced by a quasi-linear convective system — a squall line, bow echo or other line of storms much longer than it is wide — usually from a small, shallow circulation called a mesovortex along the line’s leading edge rather than from a supercell’s mesocyclone. About one U.S. tornado in five comes from one.

The pillar guide on squall lines, bow echoes and derechos introduces these tornadoes, and bow echoes on radar covers the rear-inflow jet, bookend vortices and the headline statistics. This page covers the circulations and the method NWS forecasters use to anticipate them.

What counts as a QLCS

Ashley, Haberlie and Strohm (2019, Weather and Forecasting) describe the QLCS as a type of mesoscale convective system featuring a convective line or line segments much longer than they are wide. The umbrella, following Weisman and Davis (1998), covers squall lines, leading-line/trailing-stratiform systems, line echo wave patterns and bow echoes. The label is assigned from radar reflectivity at the time of the tornado, which leaves a grey zone: a supercell embedded in a line can look linear in reflectivity.

How common, where and when

Trapp et al. (2005) put the share at 18% for 1998–2000, a figure the bow echo guide details. Their paper also found QLCS tornadoes more probable in winter than cell tornadoes, and proposed that many weak QLCS tornadoes, “perhaps as many as 12% of the total,” went unreported. NSSL’s public summary says nearly 20% of tornadoes come from QLCSs, and that they tend to be weaker and shorter-lived on average than supercell tornadoes.

Ashley et al. (2019) used machine learning on 22 years of national radar mosaics (1996–2017) to track more than 3,000 QLCSs. Across the central and eastern United States, they attribute roughly 21% of tornadoes to QLCSs. Their other findings:

• Overnight. The QLCS share rises from under 15% in the afternoon and evening to over 50% by 0600 UTC.
• Cool season. QLCS tornadoes are most frequent in April through June, but their share is highest from November through March. The winter share grew from 31.6% in 1997–2003 to 49% in 2011–2017.
• Geography. The share clusters west of the Appalachians, from the lower Great Lakes to the mid-South: 43% of tornadoes in Kentucky, 40% in Indiana, and over 35% in Alabama, Georgia, Mississippi and Tennessee.

Those are Ashley et al.’s figures, with a caveat they tested themselves: when they checked over 3,100 algorithm-identified QLCS tornadoes by hand, accuracy fell as ratings rose, to 68–87% for EF3 and above. They also note that in many areas of the mid-South, most winter and spring tornadoes come from QLCSs, in a region with relatively low warning performance.

How mesovortices form

There is no single mechanism. Ungar and Coniglio (2023, Weather and Forecasting) summarize the candidates the research has produced:

• Tilting of cold-pool vorticity. The density contrast at the edge of the outflow generates horizontal spin, which updrafts (Atkins and St. Laurent 2009) or downdrafts (Trapp and Weisman 2003; Wakimoto et al. 2015) can tip upright.
• Horizontal shearing instability. Shear along the line becomes unstable and rolls up into a series of vortices.
• Friction. Drag at the surface generates horizontal vorticity that can be tilted and stretched (Schenkman et al. 2012).
• Supercell-like processes. A descending rear-inflow jet can tilt vorticity downward, and strong low-level streamwise vorticity may be ingested much as a supercell does.

Lovell and Parker (2022) found these routes can differ even within a single QLCS.

The three-ingredients method

The most widely used operational aid comes from Jason Schaumann of the NWS in Springfield, Missouri, and R. W. Przybylinski, presented at the 26th AMS Conference on Severe Local Storms in 2012. It holds that mesovortex genesis and intensification are favored where three ingredients are co-located:

1. A part of the line where the system cold pool and the ambient low-level shear are nearly balanced or slightly shear dominant. Forecasters judge this on radar from where the updraft–downdraft convergence zone sits relative to the reflectivity updraft towers.
2. 0–3 km line-normal bulk shear of 30 kt or more (15 m/s): the component of the shear vector perpendicular to the line, which changes wherever the line bends.
3. A rear-inflow jet or enhanced outflow causing a surge or bow in the line.

The NWS Warning Decision Training Division’s one-page summary adds that 0.5° storm-relative velocity, base velocity and spectrum width can reveal a developing surge before the classic concave shape appears in reflectivity. Once all three are in place, forecasters look for additional confidence builders before warning, an approach Justin Gibbs of WDTD examines in a 2021 Journal of Operational Meteorology paper.

What testing has found

Gibbs evaluated the method on 400 samples from 74 severe weather episodes between November 2018 and July 2020, all within 90 nautical miles of a WSR-88D, including 172 tornadoes rated EF0 to EF4. He found it reasonably skillful at short lead times, especially in clearly linear systems, and found that systems with mesoscale convective vortices or supercell-like features produced tornadoes at a much higher rate than purely linear ones.

Ungar and Coniglio compared radiosonde shear with 1,820 line segments in 50 QLCSs. The shear differed significantly between segments that produced mesovortices and those that did not, but 44% of mesovortex-producing segments had line-normal 0–3 km shear at or below 30 kt. Most mesovortices formed with a rear-inflow surge present, line-normal 0–3 km shear above 10 kt and 0–6 km shear above 20 kt. The 30 kt figure is a guide, not a floor.

What to look for on radar

Gibbs consolidated the confidence builders into eleven indicators, assessed at the last volume scan before damage began. The ones readable in reflectivity and velocity include:

• A descending rear-inflow jet: enhanced velocity with lower reflectivity at the back of the line.
• A break in the 50 dBZ core along the convergence zone, or a surge or bow at mesoscale or storm scale.
• A paired inflow notch: a weak-echo notch in the line with a matching velocity signature.
• Mesovortices with rotational velocity of at least 20 kt, and bookend vortices at the ends of line segments.
• Cell mergers in the line, and any tornado in the past hour.

A tornado debris signature is not a precursor, and Gibbs excluded it for that reason, but it is confirmation. On 20 June 2021, an EF3 tornado developed quickly along the leading edge of a QLCS and crossed Naperville and Woodridge, Illinois, between about 11:02 and 11:25 pm CDT. NWS Chicago reports correlation coefficient minima co-located with tight velocity couplets, lofted debris to nearly 20,000 feet, and a second, simultaneous debris signature from an EF0 near Plainfield.

Why lead time is short

A 2014 AMS conference abstract by Stanford, Schaumann and Gagan cites studies showing that tornadoes from QLCS mesovortices develop on average only about five minutes after the mesovortex forms. A warning that waits for the circulation therefore often carries little or no lead time. Gibbs adds that traditional velocity signatures give less precursor signal for QLCS tornadoes than for supercells, and Ashley et al. describe the circulations as generally shallower and weaker, harder to detect. The overnight peak also means many occur when spotters can see little; the rain-wrapped tornado guide covers what radar offers when the tornado itself cannot be seen.

WeatherStory Radar decodes reflectivity, velocity, storm-relative velocity and correlation coefficient from raw Level II and gives the beam height of any gate you click.